The field of power generation systems using renewable energy sources comprises the conversion of energy from the sun's radiation into useful work that can then be used to generate power such as electricity. One means by which this conversion might be achieved is through that of solar heating of a working fluid such as a liquid or a gas, which fluid once heated may then be used to drive some form of turbine to generate electrical power. Systems that operate on this principle may employ large arrays of parabolic mirrors arranged in a precise manner around a solar receiver to reflect radiation from the sun on to a particular area of the solar receiver. In this manner a system is arrived at that allows a far larger amount of the sun's radiation to be directed to the solar receiver than would otherwise be practicable through enlargement of the solar receiver or some form of concentrating lens. The key factors surrounding the solar receivers are those of: efficiency of conversion between the energy of the sun's radiation and the useful work generated; cooling issues involving ensuring that the solar receiver is capable of withstanding the high temperatures that it is subjected to under focussed solar radiation; and mechanical robustness of the system in the face of operating environments, such as deserts, which often pose such issues as dust storms and ranges of temperature.
Two forms of solar receiver are direct solar receivers and indirect solar receivers. Direct solar receivers allow the solar radiation to directly pass through a window to a working fluid, which working fluid is conveniently a gas such as air. In this instance the solar radiation acts directly upon the working fluid and causes a consequent rise in thermal energy. In an indirect solar receiver system, the solar radiation is interrupted from reaching the working fluid directly by a material of some kind such as a solid surface, typically metallic, and it is this solid surface that is heated by the solar radiation and which then exchanges its heat with the working fluid via some form of thermodynamic transfer.
The indirect solar receivers have been proven to be more robust than direct solar receivers because they require no transparent material through which the solar radiation must pass in order to reach the working fluid. Such transparent material may take the form of a quartz window or similar, which is capable of withstanding high temperatures but which is nonetheless relatively fragile to environmental factors such as dust and debris, with small cracks formed thereby propagating through the window as the temperature thereof rises and thereby leading to a failure of the entire solar receiver system. In contrast, an indirect solar receiver system is advantageous because it avoids any need for these relatively fragile elements of the system, albeit at the expense of reduced rate of transfer of energy from the solar radiation to the working fluid.
Once the working fluid has been suitably heated it may then be passed through some form of heat exchanger or combustion system to further increase the temperature of the working fluid for use with an electricity generation system such as a gas turbine linked to an electrical generator
The efficiency of the system is a function of the amount of solar radiation entering the solar receiver that is effectively captured and transferred to the working fluid, followed by the efficiency of the transfer of that energy into useful work for driving the electrical generator. An issue that limits solar receivers from reaching maximum efficiency is that of re-radiation from the surface of the solar receiver back out into the atmosphere, which energy so re-radiated is lost for the purposes of power generation. It is therefore advantageous to provide a system that limits as far as possible a degree of re-radiation. A further factor in maximising the efficiency of a solar receiver is to limit the loss of thermal energy from the working fluid into its surroundings before reaching the power generation sub-system. Where the working fluid is pressurised, it is necessary to provide a pressure-tight seal around the channel through which the working fluid flows, and this pressure tight seal is difficult to create in the face of the significant temperature ranges experienced by the receiver components. Damage to the seal will lead to unwanted venting of the working fluid, which may cause damage to the solar receiver as a whole and will, at the very least, reduce the efficiency of the heat transfer process.
The present disclosure is aimed at mitigating these issues to provide a solar receiver system having improved efficiency.